The present invention relates to a combining coefficient estimation apparatus and a combining coefficient estimation method, and more particularly, to a combining coefficient estimation apparatus capable of compensating for phase noise of an OFDM system and a related combining coefficient estimation method.
Generally speaking, the orthogonal frequency division multiplexing (OFDM) technology is widely used in wireless communication system for its high special efficiency and lower multi paths distortion. The OFDM scheme divides available spectrum into large number of sub carriers and distributes communication information into some of those sub carriers as data sub carriers, other sub carriers which carry pilot information as pilot sub carriers. The pilot information contains known values and their respective received values which indicate the channel effect. An example of the usage of OFDM technology is the Digital Video Broadcasting Terrestrial (DVB-T) which has been standardized in European.
In wireless communication systems, the presence of multi paths causes fading of the received signal strength at a receive device. The fading of the received signal can cause the received signal level to drop significantly. A well-known practice of combating fading is to use a technique called “diversity”. In a generic sense, diversity is the practice of transmitting and/or receiving multiple copies of the signal and combining these copies in some optimal fashion. There are several methods of implementing diversity. One of the more popular forms of diversity is “receive spatial diversity”, in which multiple copies of the transmitted signal are received over multiple receive antennae The reason why exploiting receive spatial diversity improves receiver performance can be understood from the following line of reasoning. In a fading environment in which the fading processes are uncorrelated on the receive antennae, it is highly unlikely that all antennae of the receive array will simultaneously experience a severe drop in signal level. As a result, it is highly likely that at least one antenna element of the receive array is not in a fade, which means that at least one receive antennae is receiving a high-powered copy of the transmitted signal. The receiver will have difficulty when all receive antennae simultaneously go into a deep fade because no receive antaean can provide a high-powered version of the transmitted signal, and decoding errors will be highly likely. If the fading processes are all highly correlated, then all receive antennae will tend to go in and out of fades at the same time. When fades do occur in this case; the receiver will have difficulty decoding the signal because all antennae have faded simultaneously.
The “receive spatial diversity” requires the received signals of the multiple receive antennae combining in some optimal fashions, such as maximal ratio combining (MRC). The MRC technique combines the signals on the receive array to maximize the signal to noise ratio of the combined signal. The implementation of MRC refers to the channel effect of the communication channel. Due to the assumption of slow fading, the channel effect is stationary within several OFDM symbols. For several consecutive OFDM symbols, the transmission of pilot information estimates the channel effect of the pilot sub carriers then the interpolation technique is applied to obtain channel effect of the data sub carriers.
The frequency offset and instantaneous frequency drift of the local oscillator occurs at the receiver, which is called as phase noise of the received signals. The effect of phase noise degrades the receiving performance with two aspects as common phase error (CPE) and inter-carriers interference (ICI). The CPE causes each sub carrier signal of the OFDM symbol to shift by a fixed angle, which could be corrected by the channel estimation with pilot sub carriers. However, ICI makes the loss of orthogonal property of sub carriers, which is often regarded as the random noise effects, is hard to remove by the channel estimation.
Please refer to FIG. 1 . FIG. 1 is a frequency response of a related art least square channel estimation method. After the receiver performed channel estimation of pilot sub carriers P1, P2, P3, and P4 corresponding to the pilot signals, the frequency response of data sub carriers S1, S2, S3, S4, S5, and S6 can be obtained through linear interpolation at the frequency domain.
The effect of phase noise degrades the accuracy of the channel estimation of the pilot sub carriers according to the least square channel estimation method. The following frequency domain interpolation further degrades the channel estimation of the data sub carriers. Therefore, phase noise effect should be suppressed to upgrade accuracy of the channel estimation, thus to improve the performance of the wireless communication receiving system.